Nedal Hejazi, MD
 

 

  

                     

 

Special Aspects of modern Neurosurgery

Stereotaxy is the process by which targets visible on brain scans are accurately located during surgery. This is achieved in the classical frame-based procedures, where target localisation is done via the attachment of a device known as stereotactic frame to the head and the calculation of target co-ordinates relative to this. The technique employs a mathematical concept that identifies a point in space by its relationship to three planes intersecting at right angles to each other and intersecting at a common point; this concept forms the basis for identifying a stereotactic target by the three coordinates: anteroposterior, lateral, and vertical.

Functional Neurosurgery is the surgical alteration of brain function, classically by generating small lesions (ablative procedures) but recently by implanting stimulation electrodes. Both these methods require the precision of stereotactic methodology to succeed.

In the last few years, however, neurosurgery has entered a new and exciting era of computer-assisted surgery (interactive Image-Guided Surgery or neuronavigation). Such systems are complex and rely upon powerful computer workstations to process imaging data. They also run increasingly complex software algorithms that provide the surgeon with a degree of intraoperative information in a rapid and interactive manner resulting in neuronavigation, without the need for an attached frame.

Neuronavigation enables surgeons to find targets as small as 1mm within the brain and spinal cord. It allows the surgeon to view previous scans of the brain and indicate in real time where the surgeon is working. Three-dimensional views are created and images are presented from a surgeons eye view. Tumours may be safely and radically excised by pre-operatively definite the limits of resection; operating times could be reduced and patients could be protected from surgeon error or fatigue. The most advanced end of the spectrum would exclude the surgeon from the operation, which would be performed by a robot. This seminar will address the basic physics behind Neuronavigation as well as a brief account of the development of stereotaxis. Intraoperative problems and advantages of neuronavigation will also be assessed. A pioneer of neurosurgery, Sir Victor Hosley, experienced difficulty in placing lesions in the brains of laboratory animals. Clarke, an engineer was enlisted to help design an apparatus to locate intracranial structures. Clarke considered the brain as a three dimensional structure that could be described in terms of three axes at right angles to each other (i.e. three actnogonal axes: x;y;z). Any point in the brain can be described by a unique set of co-ordinates. This was based on Descartes system of geometry. The Horsley – Clarke frame was based on fixed anatomical landmarks that were located extra-cranially. Forty years later, a second system would be developed by Spiegel. The development of L-dopa in 1960 curtailed the growth of stereotaxy. The development of the CT scanner in 1971 by Housfield and Cormack revitalized the neurosurgeons ‘ interest in stereotaxis. This progress continued with MRI. The ability to reach a single pint in the brain reliably was important, but here was problems e.g. in a craniotomy, the frame tended to get in the way. There was a quest for frameless stereotaxis and computers provided the solution. Fiducial markers are affixed to the patient scalp and CT or MRI images are obtained. These markers stay in place until the patient has been anaesthetized and the head rigidly fixed. Using a variety of techniques, the location of the markers in space is measured and registered by a computer. As the markers are also visible on the scan, the computer is able to relate the patient’s three-dimensional space to that of the previous scan. At any stage in the operation, the surgeon can find out where he is by having the computer call up the appropriate scan. This is done by using a pointer, which also has markers on it that the computer is able to track. Systems may be classified as active or passive. An active system is the PUMA robotic arm that was pre-programmed to do specific tasks. Passive systems abound today. They may be incorporated into the microscope (Zeiss MKM) or ultilize localizing arms (ISG Wand) or optical systems (Brainlab).

VectorVision® - image-guided surgery
What is image-guided surgery?
Image-guided surgery has been employed in neurosurgery since the mid-1990s. This new technique relies on a powerful computer system, which assists the surgeon in precisely localizing a lesion, in planning each step of the procedure via a 3D model on the computer screen, and in calculating the ideal access to the tumor before the operation. The tumor and its surroundings can be viewed from different angles and in relation to landmark structures, such as the optic nerve or the brain stem. During the operating procedure, the movement of the instruments in use inside the brain can be tracked on the monitor with a precision of 1-2 millimeters, through which damage to healthy tissue and to critical areas can be avoided as much as possible.

What is VectorVision®?
VectorVision® is the state-of-the-art image-guided surgery system used for the resection of brain tumors, as well as for other neurosurgical applications. While conventional x-ray images depict tumors in two dimensions, VectorVision® provides the third dimension - depth - creating a three-dimensional image of the head and brain. This is particularly useful in reaching a tumor located deep within brain areas traditionally considered to be difficult to reach.

Thanks to pre-operative planning with VectorVision®, the procedure can be simulated on the computer: the surgeon can map out the surgical procedure ahead of time and can identify the best access to the tumor. The tumor's location and its position relative to the sensitive structures in the brain will be pinpointed so that sensitive structures can be avoided and the incision can be kept as small as possible. During the procedure, VectorVision® tracks instrument movement with an extremely high precision, providing the surgeon with total control inside the brain at all times using "real-time" imaging. The surgeon can also check if the tumor has been removed as planned. This improves the prognosis of the patient.

What types of surgery can be performed image-guided?

Stereotactic Biopsy
A stereotactic biopsy is performed in order to obtain an exact diagnosis of the tumor and to differentiate the tumor from healthy tissue or necrotic tissue. A small tissue sample is obtained from a defined region of the brain by using a long and very thin needle, which is passed through a skin puncture and tiny opening in the bone to the location of the tumor. The tissue sample is then examined by a pathologist and the nature of the lesion is determined. If the lesion proves to be a tumor, the type of tumor will be determined. The image-guided surgery system assures that the tissue sample is taken from the area of interest and not accidentally from the neighbouring region. It also helps in the pre-operative procedural planning, as well as in the avoidance of critical structures. A biopsy can be performed as part of the surgery to remove the tumor, or as a separate procedure. The risks of this procedure are low, including a less than 1% chance for significant hemmorrhage. Most patients are able to leave the hospital after a 24-hour stay.

Craniotomy
The most commonly performed surgery for the removal of a brain tumor is called a craniotomy. A craniotomy is performed to rid the patient of the tumor or to halt its progress. The objective of the surgery is to excise as much of the tumor as possible. Even if only a portion of the tumor can be removed, this can still lead to an improvement of symptoms. In most cases, the remaining tumor has to be treated by other methods. For the surgery, a portion of the scalp is usually shaved, and an incision is made through the skin. A piece of bone is removed to expose the area of brain over the tumor, then the tumor is removed. After the tumor has been removed, the bone is generally replaced and the incision closed. In a conventional craniotomy, surgeons guide themselves by what they can see, their knowledge of anatomy and their interpretation of the pre-operative scans. The greatest application of the image- guided surgery system is in anatomic navigation around tumor margins and determining regional anatomy. With the use of VectorVision®, the tumor, as well as different brain structures, can be well identified and viewed on the computer screen. The surgeon can navigate precisely to the tumor while avoiding sensible structures. The depth of the tumor can be mapped, as well as critical structures that might lie on the other side of the resection plane. Such navigation increases the precision of the procedure, and can increase the speed at which surgery can be performed.

Functional Neurosurgery
The scans taken before surgery yield a great deal of information; however, they don't always provide the precision needed to avoid critical areas of the brain during surgery. Mapping tools can improve the safety and effectiveness of surgery by locating the exact areas of the brain responsible for speech, comprehension, sensation or movement. Specific areas of the brain are stimulated by a tiny electrical current, through which their functionality can be determined. This allows these areas to be avoided during surgery.

BASIC PRINCIPLES OF INTERACTIVE IMAGE GUIDED NEUROSURGERY

  1. A method for registering the image with physical space.
  2. An intraoperative localization device.
  3. Computer video display of medical images.
  4. Methods of real time intraoperative feed back.

REGISTRATION OF IMAGE SPACE WITH PHYSICAL SPACE

CT, MRI and PET scans are obtained as 3-D volumetric data bases. There has to be registration of images with one another and with the patient’s anatomy. Registration is divided into:
 

  1. Point Methods: these define corresponding points in different images and physical space; determine their spatial co-ordinates and calculate a geometric transformation between the volumes.
  • Intrinsic points are anatomical landmarks intrinsic to the patient. T his is subject to significant inaccuracy.
  • Extrinsic points may be markers fixed to the skin and rigid markers fixed to bone.
  1. Curve and surface methods:

    Surface based registration fits sets of points extracted from contours in one image to contours of another image or the patient’s cranium. This has poorer registration accuracy than point based systems.

  2. Principal axes, correlation and interactive method: Not used for surgery.
  3. Atlas method: may be combined with extrinsic point registration.

INTRAOPERATIVE LOCALIZATION DEVICES.

Active Robotic Arm: The PUMA industrial robot was utilized after training sessions. To define its movements and algorithms . Th bulk of the PUMA proved restrictive in a surgical setting.

Passive arms: The ISG wand utilized electrogonio meters for angular detectors and counterbalamers. Optical encoders have replaced goniometers as angular sensors.

Sonic digitizer triangulation: Spark plug ultrasonic emitters wee attached to an operating microscope in a geometric configuration. Sound detectors for ultrasonic noise were arranged in a 3-D registered space. The microscope focalpoint was an end affector for the assembly. This system is sensitive to temperature and humidity.

Passive stereoscopic video: A 3-D position of an object in space is determined from its position on 2-D video images viewed from two different angles. This system is sensitive to line of sight considerations and maintenance of the spatial relationship between the video camera and the surgical space.

LED optical triangulation.

If 2 or more LED are attached to a surgical instrument, the position can be traced by 3 or more cameras. These are very accurate and flexible in usage. Line of sight considerations apply. The location of the intraoperative localizing device is registered by external fiducial markers and LED’s fixed to a base ring around the patient’s cranium.

Magnetic field guidance: A magnetic field is established by a transmitter antenna near the patient’s head and its strength is measured by a receiving antennae mounted on the surgical instrument. This system is less accurate owing to field dislocation in the theatre as well as electromagnetic interference.

Computer video display of Images.

A high resolution monitor with edor graphic overlays is the standard. The on-line re-formatting of medical images is still too slow for surgical usage and creation of segmented shaded surface and rendered images involve time delays.

3-D localization is important for "visual fusion" in the surgeon’s mind. As technology progresses, so will display of imaging.

Real-time Intraoperative feedback.

Digital scan information is still histomical and can become outdated during surgical manipulation of tissues. By digitizing the video input from intraoperative visualization, ultrasonaograpy, endoscopy, TCD and electrophysiologic recordings may be spatially registered. Intraoperative CT + MRI provide resolution imaging for real time updating of the surgical field. The problems with CT and MRI intraoperatively will be dealt with later in the seminar.

APPLICATION OF NEURONAVIGATION AND PROBLEMS ENCOUNTERED.

Brain distortion is concomitant with the practice of neurosurgery. However, the magnitude of such distortion, the influence of tumour type and the imaging characteristics that predict shift are poorly understood. The impact of brain shift on image guidance, the need for intraoperative image updating and the resolution necessary for such updating are unresolved issues.

Various techniques have been adapted to minimize this source of error:

Positioning of the craniotomy in the horizontal plane.

Avoidance of intraoperative osmotic agents.

Attendance of sites most vulnerable to displacement early in the procedure.

Placement of readiopaque beads within the tissue to enable tracking of deformation during surgery.

Roberts et al, found a number of important observation in their study of intraoperative brain shift:

The cortical surface moved as much as 1cm relative to control points.

Intraoperative movement of the brain was found to be greatest along the downward axis regardless of head position.

This was considered with the role of gravity in settling caused by unintentional CSF drainage or soft tissue removal during surgery. Positioning the head so that the craniotomy was superior and horizontal served primarily to minimize lateral shift but not reduce vertical motion. The degree of surface shift was unaffected by the size of the craniotomy.

Shift and deformation below the pial surface was not quantified in Robert’s study. Movement of deep structures is not as great as superficial landmarks, although quan measurement of absolute and relative displacement of deeper structures has been limited to the observation of Bucholz et al, using ultrasound and preoperative MRI.

Dorward et al found that during meningioma surgery, flap positioning and tumour margin delineation can be relied upon, but the deep tumour margin will be elevated toward the surgeon and encountered sooner than on the neuronavigation. Shift of deep tumour margin was less in gliomas than in meningiomas. Where a large mass was present with marked deviation of the midline, caution should be exercised in image guided resection.

BRAIN SHIFT MEASUREMENT TECHNIQUES:

Structures examined:- skull surface at the centre of the planned craniotomy.

Cortical surface at the centre of the dural opening.

-deep tumour margin

Cortical surface adjacent to the resection margin at completion.

A pointer tip was touched to a structure and the image displayed on preoperative images. When shift was present, the pointertip position appeared to lie at a distance from the chosen structure in the images. This distance was then measured with a caliper.

Displacement occurred soon after elevation of the bone flap. Image guided systems allow accurate placement of bone flaps and determination of critical cortical topography with minimum exposure, but its value declines towards the end of the case. Cranial base lesions are fixed and more rigid than the brain and are the exception in this rule.

Overcoming the problem of brain shift and managing it more adequately is the current forms of neuronavigation. Debulking of tumours, aspiration of cyst and evacuation of haematomas causes a tendency of the walls of the resultant cavity to collapse on each other, degrading the correlation between observed and originally imaged boundaries.

Ultrasonic technology is safe and easily applicable to digitizing techniques. The difficulty lies in accurately reformatting in real time, a preoperative axial data set base on ultrasongography, which is a medium of very different imaging characteristics and is obtained in small sub units. Bucholz etal have created a 2 D model of the brain, which can correct for shift on a particular plane. Intra-operatively ultrasound is coupled for a stereotactic system. Real time ultrasound is used to locate specific intracranial structure. Brain shift is demonstrated by comparing preoperative images to intraoperative images.